Halogenated copolymers of isobutylene and up to about 4 mole % of isoprene (butyl rubber) are well known polymer materials whose vulcanizates offer some outstanding properties not possessed by many other diolefin based elastomers. Articles prepared from many cured halogenated elastomers offer improved resistance to oils and greases as well as resistance to oxygen and ozone degradation. Butyl rubber vulcanizates exhibit good abrasion resistance, excellent impermeability to air, water vapor and many organic solvents, as well as resistance to aging and sunlight. These properties render these materials ideal candidates for one or more applications such as water hoses, organic fluid hoses, components in tire construction, gaskets, adhesive compositions and various molded articles.
More recently, a new class of halogenated elastomeric interpolymers have been discovered which offer many of the same properties as halogenated butyl rubber, but are even more ozone and solvent resistant and are more readily curable. These materials are the halogenation product of random copolymers of a C.sub.4 to C.sub.7 isoolefin, such as isobutylene, and a para-alkyl styrene comonomer wherein at least some of the alkyl substituent groups present in the styrene monomer units contain halogen. Preferred materials may be characterized as isobutylene interpolymers containing the following monomer units randomly spaced along the polymer chain: ##STR2## wherein R and R' are independently hydrogen, lower alkyl, preferably C.sub.1 to C.sub.4 alkyl and X is bromine or chlorine, and wherein the interpolymer is otherwise substantially free of ring halogen or halogen in the polymer backbone chain. Preferably R and R' are each hydrogen. Up to about 60 mole % of the para-alkyl styrene present in the interpolymer structure may be the halogenated structure (2) above.
Most preferred materials are elastomeric copolymers of isobutylene and para-methylstyrene containing from about 0.5 to about 20 mole % para-methyl styrene wherein up to about 60 mole % of the methyl substituent groups present on the benzyl ring contain a bromine or chlorine atom, preferably a bromine atom. These copolymers have a substantially homogeneous compositional distribution such that at least 95% by weight of the polymer has a para-alkylstyrene content within 10% of the average para-alkylstyrene content of the polymer. They are also characterized by a very narrow molecular weight distribution (Mw/Mn) of less than about 5, more preferably less than about 2.5, viscosity average molecular weights in the range of from about 300,000 up to about 2,000,000, and a glass transition temperature (Tg) of below about 50.degree. C.
These copolymers may be prepared by solution polymerization of the monomer mixture using a Lewis Acid catalyst, followed by halogenation, preferably bromination, in solution in the presence of halogen and a radical initiator such as heat and/or light and/or a chemical initiator.
Preferred brominated copolymers generally contain from about 0.1 to about 2 mole % of bromomethyl groups, most of which is monobromomethyl, with less than 0.05 mole % dibromomethyl substituents present in the copolymer. These copolymers and their method of preparation are more particularly disclosed in U.S. Pat. No. 5,162,445, the complete disclosure of which is incorporated herein by reference.
The aromatic halomethyl groups present in such copolymers permit facile cross linking to be accomplished in a variety of ways, including by means of zinc oxide or promoted zinc oxide curing systems normally used to cure halogenated butyl rubber.
Illustrative of known curing agents or accelerators which can be used alone or in conjunction with zinc oxide for curing halogenated elastomers are brominated alkyl phenol resin; N,N'-diethylthiourea; di-otho-(tolyl)guanidine salt of dicatechol borate; dipentamethylene thiuram tetrasulfide; ethylene trithiocarbamate; 2-mercaptobenzothiazole; alkyl or aryl benzothiazole disulfides, tetramethylthiuram disulfide, zinc diethyldi-thiocarbamate, zinc dibutyldithiocarbamate, and zinc dimethyldi-thiocarbamate. A preferred known cure system comprises zinc oxide and dipentamethylene thiuram tetrasulfide.
It has been found that halogenated copolymers of isobutylene and para-methylstyrene described above exhibit a very rapid onset of cure which is known as precure scorch, even more rapid than that associated with halobutyl rubber. Normally these rubbers are processed by first forming a mixture of the elastomer, filler, processing aids and other non-curative additives in a suitable mixing device such as a Banbury Mixer or two roll mill and at temperatures in the order of 75.degree. to 180.degree. C. After forming a homogeneous mixture, the composition is then prepared for vulcanization by the further mixing in of crosslinking agents such as ZnO or promoted ZnO systems, after which the curable composition is milled and sheeted out as is known in the art. It is imperative that the onset of curing in such systems (known as precure scorch) be delayed for as long as necessary to permit the processor to sheet out and shape the curable composition prior to the application of sufficient heat to properly cure the composition. It has been found that the more conventional vulcanization accelerators used with ZnO to cure halogenated isobutylene/para-methylstyrene elastomers lead to high precure scorch. These accelerators include the zinc and ammonium salts of dialkyl and diaryl dithiocarbamates.
Accordingly, it is an object of this invention to provide vulcanization systems for halogenated elastomers, particularly brominated interpolymers of isobutylene and para-methylstyrene, which exhibit low precure scorch properties.
It is a further object of this invention to provide vulcanization systems for curable halogenated elastomers which provide for the development of a full cure with minimum cure reversion and maximum cure stability.